It is quite customary in connection with tractors to have a power takeoff shaft which is driven by the engine and serves as a convenient source of power for driving various accessories. Primarily, such power takeoff shafts are coupled directly or indirectly to the engine of the tractor and the speed at which the shaft is operated is dependent upon the speed at which the engine is operated. Often, however, it is desirable to maintain a constant speed of the takeoff shaft regardless of the speed of the engine. Furthermore, this speed may be any one of two speeds. The Society of Automotive Engineers requires a nominal speed setting for a power takeoff shaft of either 540 or 1,000 rpm.
One of the problems in connection with power takeoff shafts driven by an engine is that it is occasionally very necessary to interrupt quickly the operation of the power takeoff shaft when a potentially dangerous situation arises. Often, the operator is at the rear of the tractor adjacent the power takeoff shaft and has no way of quickly shutting off the engine of the tractor. Various other conditions can arise which make it advisable to terminate quickly the operation of the power takeoff shaft.